Effects of q-profile structure on turbulence spreading: A fluctuation intensity transport analysis

Abstract

This paper studies effects of q-profile structure on turbulence spreading. It reports results of numerical experiments using global gyrokinetic simulations. We examine propagation of turbulence, triggered by an identical linear instability in a source region, into an adjacent, linearly stable region with variable q-profile. The numerical experiments are designed so as to separate the physics of turbulence spreading from that of linear stability. The strength of turbulence spreading is measured by the penetration depth of turbulence. Dynamics of spreading are elucidated by fluctuation intensity balance analysis, using a model intensity evolution equation which retains nonlinear diffusion and damping, and linear growth. It is found that turbulence spreading is strongly affected by magnetic shear s, but is hardly altered by the safety factor q itself. There is an optimal range of modest magnetic shear which maximizes turbulence spreading. For high to modest shear values, the spreading is enhanced by the increase of the mode correlation length with decreasing magnetic shear. However, the efficiency of spreading drops for sufficiently low magnetic shear even though the mode correlation length is comparable to that for the case of optimal magnetic shear. The reduction of spreading is attributed to the increase in time required for the requisite nonlinear mode-mode interactions. The effect of increased interaction time dominates that of increased mode correlation length. Our findings of the reduction of spreading and the increase in interaction time at weak magnetic shear are consistent with the well-known benefit of weak or reversed magnetic shear for core confinement enhancement. Weak shear is shown to promote locality, as well as stability. V C 2014 AIP Publishing LLC. [http://dx

Mesoscopic Transport Events and the Breakdown of Fick’s Law for Turbulent Fluxes

“…This increase in the characteristic interaction time has a stronger impact on spreading than the increase of the mode correlation length. Consequently, the overall spreading efficiency drops for low magnetic shear [66].…”

APTWG: The 4th Asia-Pacific Transport Working Group Meeting

“…The emergence of turbulence spreading is considered to be one of the fundamental processes in the non-local transport [44]. Theoretical works demonstrated that spreading length of the turbulence is maximized around intermediate magnetic shear (S. Yi) [45] and the zonal flow prevents turbulence spreading in the weak shear case (J. Li). The ballistic propagation of the turbulence front can be induced through an avalanche process related to the self-organized criticality (SOC) models [46,47].…”

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“…The mechanism most frequently involved for this is socalled turbulence spreading. [1][2][3][4][5] Turbulence spreading provides a natural way to contaminate stable domains, 6 as in the H ! L transition phenomena.…”